REYKJAVIK − An accelerating thaw of the Arctic may open vast regions for oil and gas exploration but that brings worries of spills in the fragile environment, experts said on Thursday.

Scientists behind an-eight nation report saying the Arctic sea ice could almost vanish in summer by 2100 because of global warming said offshore oil and gas operations would be easier but melting permafrost could destabilise installations on land. But oil companies are unconvinced.

"We can't say for sure whether Arctic operations will become easier or more difficult," said Mark Akhurst, climate change manager for BP, an observer at a scientists' conference in Reykjavik reviewing the Arctic report released on Monday.

"One of the big issues is... great chunks of ice shifting around," he said. "If warming creates areas where ice is far less stable then it's much more difficult to engineer."

Oil and gas is already produced around the Arctic from Alaska to Norway. Big new projects include Russia's Shtokman natural gas field in the Barents Sea, one of the world's biggest with an estimated 3.3 trillion cubic metres of gas.

"As ice recedes, resources like oil and gas will generally be easier to reach," said Arne Instanes, a Norwegian scientist who wrote a chapter of the report on infrastructure in the region.

Many environmentalists are opposed to exploration for new fossil fuels in the Arctic -- saying the burning of oil, gas and coal is already responsible for heat-trapping gases like carbon dioxide that are warming the planet.

New Treaty

"We need a new Arctic treaty to regulate access to the Arctic," said Samantha Smith, head of the WWF global conservation group's Arctic Programme. The chill Alaskan environment has yet to recover from the 1989 Exxon Valdez spill.

But Akhurst said world energy demand was likely to double or triple by 2050, and a shift to natural gas from dirtier oil or coal would help curb emissions. But even with oil at $50 a barrel, Arctic fields might cost too much.

The Arctic report, by 250 scientists from the United States, Russia, Canada, Norway, Finland, Sweden, Denmark and Iceland, says temperatures in the Arctic are rising by twice the global average and could rise by another 4-7 Celsius (7-13 F) by 2100.

The region is warming fast partly because dark ground and water, once exposed, soak up more heat than ice and snow.

A four-day conference in Iceland is reviewing all aspects of the report which covered the impact of warming on everything from polar bears to indigenous people.

"Some estimates say 25 percent of the world's oil and gas reserves are in the Arctic," said Lars-Otto Reiersen, head of the Arctic Monitoring and Assessment Programme (AMAP).

AMAP is heading a study, due in 2006, of how oil and gas may change the Arctic in the next decade. To get a benchmark of contaminants, it has taken samples of waters and sediments from places including Russia's Kara Sea to Newfoundland off Canada.

Reiersen said that, if spilt, oil was hard to mop up in the Arctic. "Spilt on ice, oil will stay frozen and when ice melts it comes out as fresh as when it went in," he said. Ice released in the spring thaw can damage plankton, birds or seals.

On land, transport is likely to become harder because ice roads will be thawed longer, trapping vehicles in mud. And buildings and oil pipelines are vulnerable to destabilisation. "There will also be problems for coastal erosion," said Instanes. Waves whipped up by storms are battering Arctic coasts that have long been protected by sheets of ice, meaning problems for building oil terminals or landfalls for pipelines.